1. Field of the Invention
The present invention relates to an active matrix liquid crystal display, and more particularly to an active matrix liquid crystal display using a nonlinear resistance element.
2. Description of the Related Art
In recent years, applications of liquid crystal displays (LCDs) centered around those of twisted nematic (TN) type have become wide spread, with a large quantity of them being utilized in the fields of wrist watches and hand calculators. On top of it, matrix type displays that can handle arbitrary display of such items as characters and graphics have also been finding their ways into industrial applications. In order to expand the application field for the matrix type LCDs, it is necessary to increase their display capacity. However, the rise of the curve for the voltage versus transmissivity characteristic is not steep enough so that, if the number of scanning lines for multiplexed drive is increased in order to enhance the display capacity, the ratio of the effective voltages that are applied respectively to a selected pixel and a nonselected pixel is reduced which gives rise to a crosstalk of an increase in the transmissivity of the selected pixel and a decrease in the transmissivity of the nonselected pixel. As a result, there is created a marked decrease in the display contrast, and the angle of visibility for which a reasonable contrast can be obtained becomes narrowed down conspicuously. For this reason, a limit of about 60 lines for the scanning lines existed in the conventional LCDs. The conventional LCD of the above kind will be referred to as a simple matrix LCD.
Now, in order to sharply increase the display capacity of a matrix type LCDs, there has been disclosed an active matrix LCD in which a switching element is arranged in series to each pixel of the LCD. As the switching element of the experimental models of active matrix LCDs announced so far, use has mostly been made of a thin film transistor (TFT) having amorphous silicon or polycrystalline silicon as the semiconductor material. On the other hand, active matrix LCDs which make use of a thin film diode (referred to as TFD hereinafter) are also drawing attention for the reason that there can be expected a simplification of the manufacturing process, an improvement in the yield and a reduction in the cost due to relatively simple manufacturing method and device structure.
Out of such thin film two-terminal element type active matrix LCD (abbreviated as TFD-LCD hereinafter), the LCD which is considered to be the closest to the practical use is that which uses a metal-insulator-metal element (abbreviated as MIM hereinafter) as the TFD. Besides MIM, a diode ring in which two amorphous pin diodes are connected in parallel with their polarities reversed to each other and a back-to-back diode in which two pin diodes are connected in series with their polarities reversed, are known as TFDs.
All of the TFDs mentioned in the above are nonlinear resistance elements in which the current increases rapidly in nonlinear fashion as the voltage applied across the ends of the element is increased. By connecting such a TFD to a liquid crystal body in series, the rise of the curve for the voltage versus transmissivity characteristic becomes steep, which makes it possible to increase the number of scanning lines.
Prior examples of LCDs that make use of such MIMs are described representatively in D. R. Baraff et al., "The Optimization of Metal-Insulator-Metal Nonlinear
Devices for Use in Multiplexed Liquid Crystal Displays," IEEE Trans. Electron Devices, Vol. ED-28, pp. 736-739 (1981) and in Shinji Morozumi et al., "250.times.240 Element LCD Addressed by Lateral MIM," Technical Report of Television Society (IPD 83-8), pp. 39-44, (issued in Dec., 1983). In addition, in patent publication gazette, they are disclosed representatively in Japanese Patent Laid Open, Gazette No. 52-149090 and Japanese Patent Laid Open, Gazette No. 55-161273 with details on the principle of operation.
In MIMs, the oxide or nitride of tantalum (Ta) or silicon is mainly used as the material for the insulator layer. Further, although almost any metal can be used as the metal in MIMs, chromium or tantalum is mainly made use of.
Out of various analytical expressions that can be employed to represent the current versus voltage (I-V) characteristic of a nonlinear resistance element, the following is known as a representative formula: EQU I=A.multidot.V.alpha. (1)
In the above expression, I is the current, V, the voltage, .alpha., a nonlinear coefficient and A is a proportionality constant. In the MIMs mentioned earlier, the value of .alpha. is 6 or greater.
Referring to FIG. 1 and FIG. 2, in a TFD-LCD, a salient electrode that is connected to a lead electrode 3 is provided on a lower glass substrate 1, an insulator film 4 is provided on the salient electrode 11, an upper electrode 5 is provided on the insulator film 4, where the upper electrode 5 is connected to a lower transparent electrode 6 which is to become a pixel On the opposite side of the lower glass substrate 1 there is disposed an upper glass substrate 7, an upper transparent electrode 9 is provided thereon, and a liquid crystal layer 10 is inserted between the lower glass substrate 1 and the upper glass substrate 7. A TFD is formed by the salient electrode 11, the insulator film 14 and the upper electrode 5.
Referring to FIG. 3, the lower transparent electrodes 6 are arranged in a lattice form, and the lower transparent electrodes 6 are joined vertically by the lead electrode 3. The upper transparent electrode 9 is provided so as to join the pixels horizontally and a pixel is formed where a lower transparent electrode 6 and an upper transparent electrode 9 are overlapped. Normally, the upper transparent electrode 9 is used as a scan signal line while the lead electrode 3 is used as a data signal line, but there may be found cases where their roles are interchanged.
An equivalent circuit for one pixel of a TFD-LCD panel may be represented in the form as shown in FIG. 4 in which a TFD 13 and a liquid crystal element 14 are connected in series, and a data signal line 15 and a scan signal line 16 are connected on both ends.
A data signal and a scan signal are applied to the data signal line 15 and the scan signal line 16, respectively, and the difference between these signal voltages becomes a voltage to be applied to the pixel. A specified row is selected by the scan signal, and only a pixel in that row to which is applied a selection signal becomes a displayable state.
FIG. 5 shows a case in which the pixel under discussion is a selected pixel, and drive signals where selected pixels and nonselected pixels exist atternately on the data signal line 15. The scan signal (a) and the data signal (b) take on the values as shown in Table 1 below in each of the positive and negative frames.
TABLE 1 ______________________________________ Negative Positive Frame Frame ______________________________________ Scan Addressed Period V.sub.P - V.sub.D -(V.sub.P - V.sub.D) Signal Nonaddressed Period 0 0 Data Selected Pixel -V.sub.D V.sub.D Signal Nonselected Pixel V.sub.D -V.sub.D ______________________________________
Here, the reason for inverting the polarity of the voltage applied to the liquid crystal between a negative and a positive values for each frame is for preventing deterioration of the liquid crystal layer. Further, the reason for applying a scan signal (V.sub.P -V.sub.D) is for making the voltage applied to the selected pixel to be V.sub.P. One picture is scanned by each one of negative and positive frame, and the display contents are written in. The addressing period T.sub.Ad is the writing interval, and the nonaddressing period T.sub.NA is the charge-holding interval. The ratio V.sub.D /V.sub.P of V.sub.D to V.sub.P is called the bias ratio which normally takes on a constant value.
A voltage (c) applied to a pixel (or pixel-applied voltage) is (data signal) minus (scan signal) which takes on the value shown in Table 2.
TABLE 2 ______________________________________ Scan Signal Addressed Nonaddressed Pixel-Applied Voltage Period Period ______________________________________ Data Selected Pixel -V.sub. P [-V.sub.D ] Signal V.sub.P [V.sub.D ] Nonselected Pixel -(V.sub.P - 2V.sub.D) [V.sub.D ] V.sub.P - 2V.sub.D [-V.sub.D ] Note The upper line is for the negative frame, and the lower line is for the positive frame. ______________________________________
The liquid crystal voltage (d) varies corresponding to the values of the voltage signal (c), generating a display contrast. Note that what is meant by the liquid crystal voltage is the voltage applied across the ends of the liquid crystal element. It should be noted that all the values for the nonaddressed period in Table 2 are given within square brackets. The meaning for this is that the voltage applied to the pixel takes on the value within the brackets depending upon the content of the data signal is selected or nonselected. The I-V characteristic of a nonlinear element should ideally be symmetric with respect to the positive and negative signs of the voltage. In an actual MIM, however, asymmetry is fairly significant as can be seen from FIG. 6. Namely, there are many cases in which the value A.sup.+ of A in Eq. (1) for V&gt;O and the value A.sup.- of A for V&lt;O are different, although .alpha. remains the same. When A.sup.- &gt;A.sup.+ holds, the absolute value of the voltage applied to the liquid crystal layer is larger for the negative frame than for the positive frame. Since the liquid crystal contrast is determined by the effective value of the liquid crystal voltage (d), flicker of the screen becomes more noticeable in such a case.